Characterization of SV40 DNA replicating molecules is being used to determine the mechanism of replication in lytically infected African green monkey kidney cells. Studies are being carried out on the mechanism of chain elongation and to characterize the proteins associated with replicating molecules. Nascent RNA chains that are attached to template DNA molecules are being isolated and regulation of viral m-RNA transcription is being studied. Transcription complexes have been isolated from infected cells which continue viral mRNA synthesis in vitro. Their properties give insight into the initial transcription products and they have shown that processing of viral transcripts is one mechanism that operates to regulate transcription.